It is pretty clear the evidence, although one should call it an observation, is that brain stimulation through change, challenging and active learning activities kept an individual's cognitive abilities higher than peers who have gone through less brain stimulations.

It's sharing this observation, not very scientific as it would need larger samples and better controlled environment but it's an interesting anecdote. Many of us perhaps see the same trend.

I think it's pretty clear that not stimulating your brain lead to drop of cognitive abilities. Parallels are made between a trainned muscle and a trainned brain. We teach kids to memorise poems to stimulate their memory. We forget about stimulation as we grow older, we tend to retire and accept the status quo that there is the active career and some day we quit and no one expect anything to be done as retired folks.

I find my own cognitive abilities dropping after a few months of slope into brain laziness. I find my cognitive speed and performance increased after a few months of rather intense focus on multiple challenging projects. I also find myself unable to perform at high cognitive levels, drop of concentration if I've been on a burn up for months without much brain rest and relaxing time. Again perhaps a good parallel with how muscles performance works.

Anecdotes are interesting, as they help see trends when in correlation with other anecdotes. I rarely hear stories about the opposite phenomenon.